This invention relates to systems for providing selectable orbital and scanning motions about a central axis, and more particularly to systems for viewing defects on the surfaces of semiconductor wafers.
A number of applications exist in which it is desirable to control a member so that it can be arbitrarily tilted in any direction about a central axis, and also to rotate it in a true orbital motion in any inclination. Usually, it is desirable that there be a large possible angle of deviation from the central axis, and that the rotary control be operable separately or together with the tilting control. Examples of such systems are found in inspection applications, in compound motion mechanisms, and in systems in which a viewing device or optical unit is to be scanned or positioned within a large field of view.
An example of a critical inspection application is the so-called bright light inspection system for semiconductor applications. Semiconductor wafers, prior to deposition or etching, have highly polished surfaces which may include minute but visually observable defects such as finger marks, scratches, and lap lines. These can commonly be discerned by visual inspection at different angles under a strong light, properly oriented. In order for an inspector to see minute defects of different kinds, the wafer must be positionable at different angles and rotatable relative to the illumination. Different conditions of operation are best for searching for specific types of defects. For example, minute lines may only appear at certain angles, and particle matter may appear to luminesce briefly when moved through the light field. A range of tilt positions and a wide number of viewing perspectives should therefore be available along with the rotary motion.
In the inspection process, it is preferred to handle the wafers by an automatic transport mechanism which extracts them from one cassette and places them at the inspection position where they can be studied in different attitudes and motions. As determined by the inspector, an examined wafer may then be passed to a different cassette or returned to the original cassette, depending upon the wafer's acceptability, and the desired direction of flow.
There are a number of available wafer handling systems that incorporate a tilt and rotate feature, sometimes solely for purposes of bright light inspection and at other times to supplement an optical inspection utilizing a microscope. These systems are, however, restricted in their capability and not readily adjustable by the operator. Obviously, it is desirable to make it as convenient as possible for the operator to select and control the angle of inclination for inspection purposes. Moreover, the wafer should be arbitrarily tiltable along two orthogonally disposed directions (e.g. roll and pitch), and through substantial angles, such as .+-.45.degree. relative to a plane normal to the axis of rotation. With such capability, the wafer can be tilted about one axis in a unidirectional scan, tilted concurrently about two axes to provide a warbling or wobbling motion, rotated in a fixed inclination about the central axis, or even tilted and rotated in a synchronized manner.
Prior art systems are limited in the fact that some can only be adjusted through a relatively small angle, some must be pre-set to a given angle, few provide a variety of control modes, and most are difficult and cumbersome to use, thus slowing the inspection process.
A reliable, precise mechanism for imparting such motions can be used in many other applications where comparable mechanical control is needed. A viewing device such as a CCD array can be directed, scanned or positioned anywhere within a wide field of view, or a robotic mechanism can be positioned in a confined space but be effective throughout a wide volume.